The Senate Foreign Relations chairman has broken with Obama on Syria and Russia. He'll be a bellwether on a nuclear deal with Iran.

Hearing President Obama's call Wednesday night for an expanded American-led military campaign in Iraq and Syria, Robert Menendez thinks back to last year. The chairman of the SenateForeign Relations Committee, a Democrat from New Jersey, pushed in 2013 to arm and train moderate Syrian rebels and for U.S. airstrikes in Syria. He steered legislation through his committee to authorize these steps, but the White House got cold feet and the initiatives died. "Come full circle," he says, "that's where...